The present invention pertains to private exchanges and, more particularly, to a basic rate private exchange for interfacing between a digital network local exchange such as the Integrated Services Digital Network (ISDN) and a wide variety of different types of subscriber station devices.
The Integrated Services Digital Network is a world-wide telephone industry standard for the delivery of digital telephony services. In its Basic Rate Interface (BRI) implementation, ISDN can utilize most existing two-wire local exchange loops. In North America, the ISDN basic rate interface provides full duplex, 160,000 bit per second service through an encoding and signaling technique known as 2B1Q. Either 2B1Q or other similar encoding techniques are used outside of North America. CCITT ISDN specifications have defined several interface reference points in the ISDN. The customer end of the two wire local exchange loop is known as the "U" reference point which connects to a network local exchange LE (FIG. 23). At the U reference point, the 160,000 bit per second bandwidth is subdivided into one 16,000 bit per second signaling and packet data D (FIG. 24) channel, two 64,000 bit per second bearer B channels and one 16,000 bit per second maintenance M channel. According to CCITT specifications, the U reference interface must be terminated by a device known as a Network Termination-1 (NT-1). The NT-1 processes the information in maintenance channel M and converts the two-wire full duplex interface into a four-wire duplex interface including two two-wire simplex interfaces. The four-wire downstream interface provided by the NT-1 is known as the "S/T" reference point. According to the CCITT reference model for ISDN, all ISDN Customer Premises Equipment connects at the S/T reference point.
Signaling activity, performing the functions of call setup, call progress and data link establishment, for BRI of the ISDN is performed by a complex protocol of, information packet exchanges across the signaling channel D between the local exchange and the customer premises equipment. This signalling activity is sufficiently complex to require the services of a powerful microcomputer and a large, complex set of software instructions contained within the customer premises equipment.
A difficulty encountered in marketing a digital network private exchange such as that connected to the ISDN U and S/T reference interface points, are totally incompatible with the existing analog Plain Old Telephone Service (POTS). None of the existing POTS customer premises equipment, such as telephones, answering machines, PBXs facsimile devices and modems can operate if they are attached directly to either the U or the S/T ISDN reference points. In the CCITT ISDN reference model, the primary method for telephony users to connect to an ISDN basic rate interface is through entirely new digital S/T reference interface customer premises equipment. Due to many factors, such as the complexity of ISDN basic rate interface signaling protocols, the digital S/T reference interface customer premises equipment has proven to be very expensive, often on the order of ten times the cost of POTS customer premises equipment. Additionally, in North America the ISDN user is responsible for the cost of supplying and installing the ISDN basic rate interface NT-1 device, and providing main power and power backup devices necessary to supply reliable DC power to the attached S/T reference interface customer premises equipment.
Over the past several years, Unites States telephone service suppliers have invested billions of dollars developing and deploying the central office switching equipment, software and human resources necessary to enable the wide spread utilization of an ISDN basic network interface. However, user acceptance of this service has been very minimal. There is now a widely accepted belief that the primary barrier to widespread user acceptance of the ISDN basic rate interface is the high cost of ISDN equipment to the ISDN customer. This cost is compounded by the incompatibility of the ISDN S/T reference interface with existing POTS customer premises equipment which prevents telephone users from utilizing their existing POTS customer premises equipment with an ISDN basic rate interface.